Heat-expandable microspheres each comprises essentially an outer shell (shell) of a thermoplastic resin and a blowing agent (core) encapsulated therein. Such heat-expandable microspheres are produced in a process, for example, suspension polymerization of a polymerizable component, in the presence of a blowing agent, in an aqueous dispersion medium comprising colloidal silica as a dispersion stabilizer (Refer to Japanese Patent Document JP-B-42-26524).
Currently, colloidal silica is an indispensable material for producing heat-expandable microspheres owing to its excellent dispersion stability. Unfortunately, colloidal silica often causes difficulty in dehydration (or filtration) of a reaction mixture after suspension polymerization, which is performed for taking out heat-expandable microspheres from the reaction mixture. In addition, colloidal silica remains on the surface of heat-expandable microspheres and it is well known that complete removal of the colloidal silica is difficult. The colloidal silica remaining on heat-expandable microspheres can cause poor adhesion between a resin and the heat-expandable microspheres in a composition comprising the resin and heat-expandable microspheres.
Recently a demand for lighter adhesive compositions is emerging, because of environmental concern and saving resin (adhesive components) in the compositions. For meeting such demand, adhesive compositions comprising hollow microspheres (plastic microballoon) manufactured by heating and expanding heat-expandable microspheres have been used, though such adhesive compositions are apt to be non-uniformly cured into materials having excessive hardness and poor elongation.
Suspension polymerization for producing heat-expandable microspheres is often performed in an acidic aqueous dispersion medium having a pH ranging from 1 to 4 when colloidal silica is used as a dispersion stabilizer. In addition, a metal reactor is usually used for the production of heat-expandable microspheres because of polymerization scale, and the use of colloidal silica inevitably causes a problem relating to the corrosion resistance of the reactor.
For solving such problem, a suspension polymerization in a neutral or basic aqueous dispersion medium comprising magnesium hydroxide or calcium phosphate as a dispersion stabilizer has been proposed (refer to Japanese Patent Document JP-A-4-292643). For improving the process with an aqueous dispersion medium comprising magnesium hydroxide, suspension polymerization in an aqueous dispersion medium further comprising a polyanion compound has also been proposed (refer to Japanese Patent Document JP-A-2011-144291).
The process in Japanese Patent Document JP-A-4-292643 further requires a dispersion-stabilizing auxiliary. Without the dispersion-stabilizing auxiliary, oil globules comprising a polymerizable component and blowing agent are not stably dispersed in an aqueous dispersion medium and the process cannot reproducibly produce heat-expandable microspheres. In addition, a nonionic emulsifier of a polyethylene glycol condensate, which has a clouding point, greatly impairs the reproducibility of suspension polymerization when it is used as the dispersion-stabilizing auxiliary. Other dispersion-stabilizing auxiliaries, such as anionic compounds including alkaline compounds of fatty acids, sulfonate salts and alkyl sulfates and polyanion compounds in Japanese Patent Document JP-A-2011-144291, also result in poor reproducibility of suspension polymerization. Those compounds react with metallic cations such as sodium from sodium chloride or magnesium from magnesium chloride added to the aqueous dispersion medium to deteriorate the surface-active property of the medium and the dispersion stability of the oily globules. Thus the processes in Japanese Patent Document JP-A-4-292643 and Japanese Patent Document JP-A-2011-144291 produce heat-expandable microspheres with low encapsulation efficiency of blowing agents and the resultant heat-expandable microspheres have poor heat-expansion performance.